1. Field of the Invention
The present invention relates to an oil pressure control system for an automatic transmission system for use with automobiles.
2. Description of the Prior Art
In an automatic transmission system which includes a fluid torque converter, a transmission gear and friction engaging means for establishing a selected transmission engagement in said transmission gear. The automatic transmission system is controlled by an oil pressure control system, when a manual shift valve, which is operated by a manual change lever is shifted to the D range, a plurality of speed shift valves incorporated in the oil pressure control system are automatically shifted according to increase or decrease of the vehicle speed. This shifting alters the engaging condition of said friction engaging means, thereby establishing a selected transmission engagement in said transmission gear, generally between three or four forward speed ranges. By contrast, when the rate of acceleration or deceleration is to be particularly increased according to the desire of the driver, the manual change lever is shifted to a low speed range such as 3, 2 or 1 range. When the manual change lever is shifted to one of these low speed ranges, said oil pressure control system is restricted so that automatic shifting of said transmission gear beyond the particular speed range set out by the manual change lever do not occur even when the vehicle speed has exceeded a predetermined speed level. However, the purpose of providing the low speed ranges is to accomplish a required acceleration or deceleration rate within a specific limit within which the vehicle is operated in a normal condition. In other words, if the automatic shifting of the transmission system is restricted even when the operating condition of the vehicle has exceeded a limit for normal operation, there is a danger that the rotational speed of the engine may become extroadinarly high. This may occur, for example, when a vehicle runs on a very steep downhill slope, thereby causing damage to the engine.